Add basic support for OpenGraph header tags for public installs

Summary: Ref T13018. This is easy to get working roughly, at least, and seems reasonable.

Test Plan: Viewed page source, saw tags. Custom header logo still worked. Pretty hard to debug against a local install since Disqus / debugger tools can't hit it, but I'll see what it looks like in production and tweak it if I got anything horribly wrong.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13018

Differential Revision: https://secure.phabricator.com/D18780